Safety Augmentation for Volitional Human Locomotion via Lower-Limb Exoskeletons: A Case Study

User safety is a crucial factor to consider when designing control paradigms for lower-limb exoskeletons. Existing control paradigms mainly focus on providing assistance for human users under stable walking conditions, ignoring situations that human users may lose their balance due to external perturbations during locomotion. In this letter, we propose a safety framework for lower-limb exoskeletons to augment safety for volitional human motion based on Control Barrier Functions. The safety indicators are defined as the human's center of mass and swing foot position lying within self-selected ranges. Instead of enforcing reference trajectories, we incorporate human inputs and preferences in a two-layer quadratic program structure based on Control Barrier Functions to generate assistance for ensuring safety. Simulation results on a human wearing an exoskeleton demonstrate that the proposed control paradigm can generate assistance to assist human users in maintaining balance while undergoing gait perturbations and recovering from initial unsafe postures.